Sequence of SOHO-EIT first difference images showing the intensity (density)
enhancement and following rarefaction associated with a shock wave expanding
across the solar disk from the site of the origin of a coronal mass ejection
(CME), 1997 May 12. A halo CME was observed by SOHO-LASCO. These images were
formed from the differences of successive images in the emission lines of
Fe XII near 195 A; this ion is formed at temperatures of about 1.5 million
degrees. The wave front travels at speeds of ~ 300 km/s, typical of a fast
mode Alfvén shock in the lower solar corona.